Electromagnetic (EM) waves from charged particles at relativistic speeds accelerating in a magnetic field, e.g., to follow a curved path, are called synchrotron radiation. Relativistic time contraction bumps the EM frequency observed in the lab from the frequency corresponding to energies of Giga electron Volts (GeV, 1 GeV=109 electron Volts) for electrons, into the X-ray range of kilo electron Volts (keV, 1 keV=103 electron Volts). Another effect of relativity is that the radiation pattern is distorted from an isotropic dipole pattern expected from non-relativistic theory into an extremely forward-pointing cone of radiation. This makes artificial synchrotron radiation the brightest known source of X-rays. The planar acceleration geometry of a synchrotron makes the radiation linearly polarized when observed in the orbital plane, and circularly polarized when observed at a small angle to that plane.
Since the discovery of synchrotron radiation, analytic techniques have been continually developed to exploit its unique properties as a source of X-rays. Undulators and wigglers are insertion devices which are inserted into a straight section of a synchrotron or a storage ring specifically to generate synchrotron radiation with particular characteristics. Many state-of-the-art methods pursue the advantages of high flux and polarization of synchrotron radiation.
It is noted that many methods would otherwise be impossible without good energy resolution—the ability to select a narrow band of EM frequencies (or corresponding EM wavelengths) from the synchrotron radiation. As is well known, the quantized energy (photon) of an EM beam is proportion to its EM frequency. One such technique that relies on good energy resolution is multi-wavelength anomalous diffraction (MAD), which takes advantage of differences in anomalous signals at carefully selected x-ray wavelengths to determine the phases of x-rays diffracted by protein crystals and thus to determine the structure of those crystals. A device that transmits a mechanically selectable narrow band of wavelengths of EM radiation chosen from a wider range of EM wavelengths available at the input is called a monochromator.